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Abstract

In recent years, a number of natural products isolated from Chinese herbs have been
found to inhibit proliferation, induce apoptosis, suppress angiogenesis, retard metastasis
and enhance chemotherapy, exhibiting anti-cancer potential both in vitro and in vivo. This article summarizes recent advances in in vitro and in vivo research on the anti-cancer effects and related mechanisms of some promising natural
products. These natural products are also reviewed for their therapeutic potentials,
including flavonoids (gambogic acid, curcumin, wogonin and silibinin), alkaloids (berberine),
terpenes (artemisinin, β-elemene, oridonin, triptolide, and ursolic acid), quinones
(shikonin and emodin) and saponins (ginsenoside Rg3), which are isolated from Chinese medicinal herbs. In particular, the discovery of
the new use of artemisinin derivatives as excellent anti-cancer drugs is also reviewed.

Background

Surgery, chemotherapy and radiotherapy are the main conventional cancer treatment
often supplemented by other complementary and alternative therapies in China [1]. While chemotherapy is one of the most extensively studied methods in anti-cancer
therapies, its efficacy and safety remain a primary concern as toxicity and other
side effects of chemotherapy are severe. Moreover, multi-drug resistant cancer is
even a bigger challenge. Medicinal herbs are main sources of new drugs. Newman et al. reported that more than half of the new chemicals approved between 1982 and 2002 were
derived directly or indirectly from natural products [2]. Some active compounds have been isolated from Chinese medicinal herbs and tested
for anti-cancer effects. For example, β-elemene, a compound isolated from Curcuma wenyujin Y. H. Chen et C. Ling (Wenyujin), is used as an anti-cancer drug in China. For this study, we searched
three databases, namely PubMed, Scopus and Web of Science, using keywords "cancer",
"tumor", "neoplastic" and "Chinese herbs" or "Chinese medicine". Publications including
research and review papers covered in this review were dated between 1987 and 2011,
the majority of which were published between 2007 and 2011. Chinese herb-derived ingredients,
including flavonoids, alkaloids, terpenes, quinones and saponins, were found.

Gambogic acid (GA)

GA (Figure 1A) is the principal active ingredient of gamboges which is the resin from various Garcinia species including Garcinia hanburyi Hook.f. (Tenghuang) [3]. GA has various biological effects, such as anti-inflammatory, analgesic and anti-pyretic
[3] as well as anti-cancer activities [4,5]. In vitro and in vivo studies have demonstrated its potential as an excellent cytotoxicity against a variety
of malignant tumors, including glioblastoma, as well as cancers of the breast, lung
and liver. GA is currently investigated in clinical trials in China [6-8].

GA induces apoptosis in various cancer cell types and the action mechanisms of GA
remain unclear. Transferrin receptor (TfR) significantly over-expressed in a variety
of cancers cells may be the primary target of GA [4]. The binding of GA to TfR in a manner independent of the transferrin binding site,
leading to the rapid apoptosis of tumor cells [4]. Proteomics analysis suggests that stathmin may be another molecular target of GA
[9]. The importance of the role of p53 in GA-induced apoptosis remains controversial
[5,10]. Furthermore, GA antagonizes the anti-apoptotic B-cell lymphoma 2 (Bcl-2) family
of proteins and inhibits all six human Bcl-2 proteins to various extents, most potently
inhibiting myeloid cell leukemia sequence 1 (Mcl-1) and Bcl-B, as evidenced by a half
maximal inhibitory concentration (IC50) lower than 1 μM [11]. Moreover, GA also influences other anti-cancer targets, such as nuclear factor-kappa
B (NF-κB) [12] and topoisomerase IIα [13].

Notably, the combination of GA with other compounds enhances their anti-cancer activities
[15-17]. For example, He et al. [15] reports that proliferative inhibition and apoptosis induction are much more visibly
increased when Tca8113 cells are treated with combined GA and celastrol, indicating
that the combination of GA and celastrol can be a promising modality for treating
oral squamous cell carcinoma. Another study showed that GA in combined use with 5-fluorouracil
(5-FU) induced considerably higher apoptosis rates in BGC-823 human gastric cells
and inhibited tumor growth in human xenografts [16]. Furthermore, low concentrations of GA were found to cause a dramatic increase in
docetaxel-induced cytotoxicity in docetaxel-resistant BGC-823/Doc cells [17]. Magnetic nanoparticles of Fe3O4 (MNPs-Fe3O4) were reported to enhance GA-induced cytotoxicity and apoptosis in K562 human leukemia
cells [18].

Curcumin induces non-apoptotic cell death, such as autophagic cell death, which involves
the degradation of the cell's own components through lysosomal machinery [23]. In vitro and in vivo studies have demonstrated that curcumin induces autophagic cell death, as evidenced
by the immunoreactivity of microtubule-associated protein light chain 3 (LC3) in myeloid
leukemia cells. The action mechanism is attributed to the inhibition of the Akt/mammalian
target of rapamycin/p70 ribosomal protein S6 kinase pathway and activation of extracellular
signal-regulated kinase 1/2 by curcumin in malignant glioma cells [26,27]. In addition, autophagic inhibitor bafilomycin A1 suppresses curcumin-induced cell
death [28]. Another type of non-apoptotic cell death induced by curcumin is paraptosis which
is observed in malignant breast cancer cells but not in normal breast cells. Curcumin
induces paraptotic events (eg the promotion of vacuolation accompanied with mitochondrial and/or endoplasmic reticular
swelling and fusion) and decreases the level of paraptotic inhibitor protein AIP-1/Alix
[29]. These paraptotic events are attributed to superoxide anion and proteasomal dysfunction
[29].

Clinical trials have been or are currently being conducted to evaluate the tolerance,
safety, pharmacokinetics and efficiency of curcumin as well as its combination therapy
with current anti-cancer drugs [39]. A phase I clinical trial found no dose-limiting toxicity in patients treated with
an oral-dose of up to 8g/day of curcumin. The recommendation is seven consecutive
doses (6g/day) of curcumin every three weeks in combination with a standard dose of
docetaxel [40]. Improvements in biological and clinical responses were observed in most treated
patients [40]. A phase II trial of gemcitabine-resistant pancreatic cancer found chemotherapeutic
drugs in combined use with curcumin to be sufficiently safe, feasible and efficient.
While the bioavailability of curcumin is relatively poor, two out of 21 patients in
the phase II trial showed clinical biological responses; one patient exhibited marked
tumor regression coupled with a significant increase in serum cytokine levels [41,42].

Wogonin

Wogonin (Figure 1C) is one of the flavonoids isolated from Scutellaria baicalensis Georgi (Huangqin), with its dry herb weight consisting of up to 0.39 mg/100 mg of wogonin [43]. Wogonin has been widely used in the treatment of various inflammatory diseases owing
to its inhibition of nitric oxide (NO), prostaglandin E2 and pro-inflammatory cytokines production, as well as its reduction of cyclooxygenase-2
(COX-2). In vitro studies [44-48] have shown wogonin to possess cytostatic and cytotoxic activities against several
human tumor cell lines.

Wogonin induces apoptosis through the mediation of Ca2+ and/or inhibition of NF-κB, shifting O2- to H2O2 to some extent; H2O2, in turn, serves as a signaling molecule that activates phospholipase Cγ. Ca2+ efflux from the endoplasmic reticulum is then regulated, leading to the activation
of Bcl-2-associated agonist of cell death [44]. Wogonin may also directly activate the mitochondrial Ca2+ channel uniporter and enhance Ca2+ uptake, resulting in Ca2+ overload and mitochondrial damage [44]. Furthermore, wogonin induces cell type-dependent cell cycle inhibitions in cancer
cells, such as those observed in human cervical carcinoma HeLa cells at the G1 phase [48] and in THP-1 cells at the G2/M phase [46] respectively. Unlike the inhibitory effect of baicalein and baicalin on normal human
fetal lung diploid TIG-1 cells [46], wogonin imposes minor or almost no toxicity on normal peripheral T cells [44], TIG-1 cells [46] and human prostate epithelial cells [47]. This selective inhibition of wogonin is due to a high expression of L-type voltage
dependent Ca2+ channels in cancer cells [44]. In addition, wogonin suppresses VEGF-stimulated migration and tube formation in
HUVEC by inhibiting VEGF receptor 2 (VEGFR2) instead of VEGFR1 phosphorylation [49].

The synergistic effect of wogonin on chemotherapy drugs, such as etoposide, has also
been investigated. Wogonin significantly improves etoposide-induced apoptosis in cancer
cells in a similar capacity as the typical P-glycoprotein (P-gp) inhibitors verapamil
and cyclosporine A [50-52]. However, other P-gp substrates, such as doxorubicin and vinblastine, do not show
any synergistic effect [52]. Similar effect was also found when combination treatment with 5-FU in human gastric
MGC-803 cells and in MGC-803 transplanted nude mice [53]. The underlying mechanisms might be due to its pro-apoptotic effect and inhibition
of NF-κB nuclear translocation activity [53].

Anti-inflammatory and anti-viral activities of wogonin may also contribute to tumor
prevention [54]. Wogonin is a good anti-cancer candidate due to its broad toxicities to various types
of tumor cell lines and the low toxicities to normal tissues, as well as the synergistic
effects.

Silibinin

Silibinin (Figure 1D), a mixture of flavonoids derived from Silybum marianum (Shuifeiji), is therapeutically used for the treatment of hepatic diseases in China, Germany
and Japan. Silibinin has effects on many cancers, such as prostate, colon, bladder
and lung cancers [55,56], particularly the migration, invasion and metastasis of cancer cells [57]. In a transgenic adenocarcinoma of the mouse prostate (TRAMP) mouse model, silibinin
inhibits tumor growth, progression, local invasion and distant metastasis [56]. Silibinin induces both death receptor-mediated and mitochondrial-mediated apoptosis
in human breast cancer MCF-7 cells [58]. Silibinin also reduces hepatocellular carcinoma xenograft growth through the inhibition
of cell proliferation, cell cycle progression, as well as phosphatase and tensin homolog/P-Akt
and extracellular signal-regulated kinase (ERK) signaling. These effects induce apoptosis
and increase histone acetylation and superoxide dismutase-1 (SOD-1) expression on
human hepatocellular carcinoma xenografts [59]. Not only does silibinin inhibit primary prostatic tumor progress but also protects
against angiogenesis and late-stage metastasis. Therefore, silibinin may have a potential
for improving survival and reducing morbidity in prostate cancer patients [60].

The combined use of silibinin with 1,25-dihydroxyvitamin D3 promotes the expression
of both differentiation-promoting and -inhibiting genes in acute myelogenous leukemia
cells and the latter can be neutralized by a highly specific pharmacological inhibitor,
suggesting the therapeutic potential of silibinin [64].

Berberine

Berberine (Figure 1E) is an isoquinoline alkaloid isolated from Coptidis Rhizoma (Huanglian), which is a Chinese medicinal herb for heat dissipation and detoxification, with
its dry herb weight consisting of up to 7.1 mg/100 mg of berberine [65]. Berberine has diverse pharmacological activities [66-70] and is especially used as an antibacterial and anti-inflammatory gastrointestinal
remedy in China [71]. Berberine has anti-proliferative effects on cancer cells has been documented [72-78]. Multiple targets of berberine have been identified, including mitochondria, DNA
or RNA, DNA topoisomerases, estrogen receptors, MMPs, p53 and NF-κB [74,79-82]. Berberine exerts cytotoxicity and inhibits telomerase and topoisomerase in cancer
cells by specifically binding to oligonucleotides or polymorphic nucleic acid and
by stabilizing DNA triplexes or G-quadruplexes [81,83,84]; the electrostatic interactions may be quantified in terms of the Hill model of cooperative
interactions [85].

In addition to apoptotic alteration induced by berberine, recent findings are about
anti-cancer mechanisms that have a higher propensity to cause autophagy. Berberine
induces autophagic cell death in human hepatocellular liver carcinoma cell lines (HepG2)
and MHCC97-L cells, which may be diminished by cell death inhibitor 3-methyladenine
through beclin-1 activation and mammalian target of rapamycin (mTOR) signaling pathway
inhibition [90]. In addition, berberine also modifies LC3, an autophagic marker, in human lung cancer
A549 cells, indicating that autophagy may play a crucial role in berberine-induced
cancer cell death [91].

Artemisinin and its derivatives (ARTs)

Artemisinin (Figure 1F) is an active terpene of the Chinese medicinal herb Artemisia annua L. (Huanghuahao) used in China to treat malaria and fever. ARTs, such as dihydroartemisinin (DHA)
and artesunate (Figure 1G), exhibit anti-cancer activities in vitro and in vivo [103-106]. DHA is one of the main metabolites of ARTs and artesunate is a semi-synthesized
derivative of ARTs; both compounds exhibit anti-cancer potentials.

The anti-cancer potential of ARTs has been demonstrated in various cancer cells including
those of leukemia and other cancer cells of breast, ovary, liver, lung, pancreas and
colon [104,105]. The selective anti-cancer potential of ARTs was related with the expression of different
molecules such as c-MYC, cdc25A, EGFR, γ-glutamycysteine synthetase (GLCLR) [105,106]. ARTs also exert anti-cancer effects in vivo in multiple cancer types [103,107,108]. For example, either DHA or artesunate has anti-cancer activity against pancreatic
cancer xenografts [107,109].

The anti-cancer mechanism of ARTs is likely to be related to the cleavage of the iron-
or heme-mediated peroxide bridge, followed by the generation of reactive oxygen species
(ROS) [110-112]. According to Efferth et al. [113], CCRF-CEM and U373 cells are sensitive to a combined treatment of ARTs and iron (II)-glycine
sulfate or holotransferring. Pretreatment with deferoxamine mesylate salt (an iron
chelator) visibly reduces DHA-induced apoptosis in HL-60 leukemia cells [104]. The anti-cancer potential of ARTs is possibly connected to the expression of TfR.
The synergism of artesunate and iron (II)-glycine sulfate co-treatment is unsuitable
for all types of tumor cells [114]. Endoplasmic reticulum stress is partially involved in some cases of ARTs-mediated
anti-proliferation [115,116].

ARTs induce cell cycle arrest in various cell types [103,115,117]. For example, DHA and artesunate effectively mediate G1 phase arrest in HepG2 and
Hep3B cells [103]. DHA reduces cell number in the S phase in HCT116 colon cancer cells [115]. Interestingly, DHA also arrests the G2 phase in OVCA-420 ovarian cancer cells [117]. Thus, ART-mediated cell cycle arrest is possibly cell type dependent. ARTs also
induce apoptotic cell death in a number of cell types, in which the mitochondrial-mediated
apoptotic pathway plays a decisive role [104,106]. For instance, DHA enhances Bax and reduces Bcl-2 expression in cancer cells [103,107]. DHA-induced apoptosis is abrogated by the loss of Bak and is largely reduced in
cells with siRNA-mediated downregulation of Bak or NOXA [118]. However, DHA activates caspase-8, which is related to the death receptor-mediated
apoptotic pathway in HL-60 cells [104]. DHA enhances Fas expression and activates caspase-8 in ovarian cancer cells [119]. DHA also enhances death receptor 5 and activates both mitochondrial- and death receptor-mediated
apoptotic pathways in prostate cancer cells [120]. ARTs-induced apoptosis in cancer cells may involve p38 MAPK rather than p53 [103,104].

ARTs inhibit angiogenesis which is a vital process in metastasis [121-124]. DHA inhibits chorioallantoic membrane angiogenesis at low concentrations and decreases
the levels of two major VEGF receptors on HUVEC [122]. Conditioned media from K562 cells pre-treated with DHA inhibits VEGF expression
and secretion in chronic myeloid leukemia K562 cells, leading to angiogenetic activity
decrease [121,124]. Artemisinin inhibits cell migration and concomitantly decreases the expression of
MMP2 and the αvβ3 integrins in human melanoma cells [125]. ARTs also regulate the levels of u-PA, MMP2, MMP7 and MMP9 all of which are related
to metastasis [126].

ARTs exert synergistic effects with other compounds. Combination of DHA and caboplatin
significantly reduces the development of ovarian cancer as compared with DHA only
[119]. Combined use of DHA or artesunate with gencitabine inhibits the growth of HepG2
and Hep3B transplanted tumors [103]. Supra-additive inhibition of cell growth in some glioblastoma multiforme cells is
observable when artesunate is in combined use with EGFR inhibitor OSI-774 [127]. DHA not only up-regulates death receptor 5 expression but also cooperates with TNF-related
apoptosis-inducing ligand (TRAIL) to induce apoptosis in human prostate cancer cells
[120]. Therefore, either used alone or in combination with other compounds, ARTs are promising
compounds for chemotherapy.

β-elemene

Elemene (Figure 1H) is a sesquiterpene mixture isolated from more than 50 Chinese herbs and plants,
such as Curcuma wenyujin Y. H. Chen et C. Ling (Wenyujin) [128]. Elemene is mainly composed of β- and δ- and γ-elemene, with β-elemene accounting
for 60%-72% of all three isoforms. β-elemene exerts anti-cancer potential in brain,
laryngeal, lung, breast, prostate, cervical, colon and ovarian carcinomas [128-130]. Elemene shows synergistic effects in combination with other chemotherapeutic drugs
[131], leading to the blockade of cell cycle progression by modulating the G2 cell cycle
checkpoint and inducing G2/M arrest in human non-small cell lung cancer (NSCLC) and
ovarian carcinoma cells while inducing G0/G1 phase arrest in glioblastoma cell lines
through phosphorylation of p38 MAPK [129,130,132]. In NSCLC cells, β-elemene induces cell arrest at the G2/M phase by increasing phospho-Cdc2
(Tyr15) and p27/Kip1, and by decreasing phospho-Cdc2 (Thr161) and cyclin B1. Moreover,
elemene reduces the expression of Cdc25C, activates Cdc2 and increases Chk2 [129]. β-elemene combined with cisplatin also mediate G2/M cell cycle arrest in chemo-resistant
ovarian carcinoma cells through down-regulation of cyclin B1 and Cdc2 by elevating
the levels of phosphorylation of Cdc2, Cdc25C, p53, p21/Waf1, p27/Kip1 and GADD45
[130]. β-elemene also induces mitochondrial-mediated apoptosis in prostate cancer and NSCLC
cells [128,129]. Combining β-elemene with cisplatin, docetaxel and taxanes significantly increases
its inhibitory effect in androgen-independent prostate carcinoma DU145 and PC-3 cells,
as well as in NSCLC H460 and A549 cells [131]. β-elemene enhances cellular uptake of taxanes due to the alteration of cell membrane
permeability may partly account for its synergistic effects with taxanes [131]. Elemene inhibits the growth of human epidermoid and thyroid cancer cells in vivo [133], and passes through the blood-brain barrier [134], suggesting its potential for treating cerebral malignancy.

β-elemene has been approved by China's State Food and Drug Administration as a second
class innovative drug and is prescribed as an adjuvant drug for some tumor therapies
in China.

Oridonin

Oridonin (Figure 1I) is a diterpenoid isolated from Rabdosia rubescens (Hemsl.) Hara (Donglingcao), with its dry raw herb consisting of up to 0.35% of oridonin [135]. Rabdosia rubescens (Hemsl.) Hara has long been used to treat sore throat, tonsillitis, and esophageal
cancer by native residents of Henan Province. Oridonin was included in the Chinese
Pharmacopoeia in 1977. Main chemical constituents of Rabdosia rubescens (Hemsl.) Hara are ent-Kaurene diterpenoids, which have multiple biological activities,
such as anti-inflammatory, anti-bacterial and anti-tumor effects.

Oridonin significantly inhibits tumor cell proliferation, induces cell cycle arrest
and promotes cell death. In anti-proliferation tests, different cell lines exhibited
similar sensitivity to oridonin with an IC50 of about 40-80 μM after 24 hours of treatment [136-141]. Oridonin induces G2/M cell cycle arrest by up-regulation of heat shock 70 kDa protein
1, serine-threonine kinase receptor-associated protein, translationally controlled
tumor protein, stress-induced phosphoprotein 1, trifunctional purine biosynthetic
protein adenosine-3 and inorganic pyrophosphatase as well as down-regulation of poly(rC)-binding
protein 1 [142] in a p53-independent and p21/Waf1-dependent manner [143]. Induction of apoptosis contributes to oridonin-induced cell death, mainly through
mitochondrial-mediated pathways. The up-regulation of Fas, Fas ligand (FasL) and Fas
(TNFRSF6)-associated via death domain (FADD) expression, as well as the down-regulation of pro-caspase-8 expression
suggests that the activation of the Fas/FasL pathway may also be partially involved
in oridonin-induced apoptosis [144]. Possible downstream responses include the induction of loss of mitochondrial transmembrane
potential [145], the activation of several caspases [136,146], the down-regulation of Bcl-2, the up-regulation of Bax and Bid [136,147] as well as the promotion of cytochrome c release [147] and PARP cleavage [148]. However, the regulation of Bcl-xL and participation of caspase-3/9 remain controversial
[136,143,146,148-150]. Oridonin-induced intracellular ROS formation may be an initiator of this process
[143,151]. Other proteins may also be involved in oridonin-induced cell cycle arrest and apoptosis;
these proteins include ERK [144,152], p38MAPK [149], insulin-like growth factor 1 receptor [153], EGFR [154], NF-κB [155], as well as p16, p21/Waf1, p27/Kip1 and c-MYC [156]. Oridonin induce cell death by affecting the balance of apoptosis and necrosis. In
A375-S2 cells, low concentrations (34.3 μM) of oridonin induce p53 and ERK-dependent
apoptosis whereas high concentrations (137.4 μM) induce necrosis [146]. In L929 cells, oridonin induces a caspase-independent and mitochondria- or MAPK-dependent
cell death through both apoptosis and necrosis [139,149]. Similar results are also observed in A431 cells [154]. Oridonin also induces simultaneous autophagy and apoptosis in MCF-7 [157] and HeLa cells [138]. This autophagy may be attributed to the inactivation of Ras, changes in mitochondrial
membrane potential [158], activation of PKC, Raf-1 or c-jun N-terminal kinase (JNK) signaling [141] and even NF-κB signaling pathways [159]. Inhibition of autophagy is attributed to apoptotic up-regulation because oridonin-induced
apoptosis augmentation is accompanied by reduced autophagy [138] whereas oridonin-induced autophagy inhibits ROS-mediated apoptosis by activating
the p38 MAPK-NF-κB survival pathways in L929 cells [160]. Oridonin inhibits DNA, RNA, and protein syntheses [161], decrease telomerase, as well as down-regulate human telomerase reverse transcriptase
mRNA expression [162]. The in vivo anti-tumor activities of oridonin have been demonstrated in different tumors such
as Ehrlich ascites carcinoma, sarcoma-180 solid tumors and in leukemic mice models
[163,164].

Triptolide is active in pro-apoptosis in diverse tumor cell types including ovarian
cancer [166], myeloma [167], myeloid leukemia [168], thyroid carcinoma [169] and pancreatic tumor cells [170]. Many in vitro and in vivo studies have tried to elucidate the potential mechanism of triptolide; however, conclusions
have been inconsistent. Triptolide seems to induce apoptosis via different pathways in various cell lines. For example, triptolide induces apoptosis
by the overexpression of cytomembrane death receptor in a caspase-8-dependent manner
in pancreatic tumor [170] and cholangiocarcinoma cells [171]. Triptolide also promotes apoptosis in leukemic and hepatocarcinoma cells by the
mitochondrial-mediated pathway [172,173].

Triptolide is a potent inhibitor of tumor angiogenesis in a zebrafish embryo model
and demonstrates potent activities against vessel formation by nearly 50% at 1.2 μM
[165]. In a xenograft model, triptolide (0.75 mg/kg/day) blocks tumor angiogenesis and
progression in a murine tumorigenesis assay possibly correlated with the down-regulation
of proangiogenic Tie2 and VEGFR-2 expression [174]. In vitro studies have shown that triptolide inhibits the proliferation of HUVEC. A chick embryo
chorioallantoic membrane test shows that triptolide inhibits angiogenesis as well.
Triptolide impairs VEGF expression in thyroid carcinoma TA-K cells and down-regulates
NF-κB pathway activity; the target genes of triptolide are associated with endothelial
cell mobilization in HUVEC [165]. The down-regulation of NF-κB signaling [175], in combination with the inhibition of VEGF expression [176], may be the anti-angiogenesis action of triptolide.

Furthermore, triptolide inhibits tumor metastasis, reducing basal and stimulated colon
cancer cell migration through collagen by 65% to 80% and decreasing the expression
of VEGF and COX-2 [174]. Triptolide inhibits the expression of multiple cytokine receptors potentially involved
in cell migration and cancer metastasis, including the thrombin receptor, CXCR4, TNF
receptors and TGF-β receptors [174]. Triptolide also inhibits interferon-γ-induced programmed death-1-ligand 1 surface
expression whose up-regulation is an important mechanism of tumor immune evasion in
human breast cancer cells [177]. Triptolide inhibits the experimental metastasis of melanoma cells to the lungs and
spleens of mice [178]. Moreover, triptolide inhibits the migration of lymphoma cells via lymph nodes, a result which may be related to its anti-proliferative effects and blockage
of the SDF-1/CXCR4 axis [179].

Triptolide enhances the anti-neoplastic activity of chemotherapy [180,181]. The combination index-isobologram indicates that the effect of triptolide on 5-FU
is synergistic on colon carcinoma [180]. In a tumor xenograft model, the combined effects of triptolide (0.25 mg/kg/day)
and 5-FU (12 mg/kg/day) on the growth of colon carcinoma are superior to those of
individual agents [180]. Triptolide is synergistic with other anti-cancer agents or therapies including hydroxycamptothecin
[181], idarubicin, AraC [182], TRAIL [183] and ionizing radiation [184]. These results indicate the therapeutic potential of triptolide in treating cancer.

UA induces apoptosis via both extrinsic and intrinsic signaling pathways in cancer cells [189]. In PC-3 cells, UA inhibits proliferation by activating caspase-9 and JNK as well
as FasL activation and Akt inhibition [190]. A significant proliferation inhibition and invasion suppression in both a dose-
and time-dependent manner is observed in highly metastatic breast cancer MDA-MB-231
cells; this inhibition is related to the down-regulation of MMP2 and u-PA expression
[191]. Moreover, UA reduces IL-1β- or TNF-α-induced rat C6 glioma cell invasion and inhibits
the interaction of ZIP/p62 and PKC-ζ [192]. Nontoxic UA concentrations inhibit vessel growth in rat aortic ring and down-regulate
matrix MMPs such as MMP2 and MMP9 [193]. In other cancer cell lines, such as Hep3B, Huh7 and HA22T cells, UA exerts a potential
anti-angiogenic effect by decreasing HIF-1α, VEGF and IL-8 gene expression [194].

Shikonin

Shikonin (Figure 1L) is a natural anthraquinone derivative isolated from the roots of Lithospermum erythrorhizon (Zicao) and exerts anti-tumor effects mainly by inhibiting cell growth and inducing apoptosis.
The underlying molecular mechanisms vary with cell types and treatment methods. Shikonin
induces apoptosis in a classic caspase-dependent pathway in cervical, bladder and
melanoma cancer cells [195-198]. Shikonin induces necroptosis regardless of the drug concentration in caspase-3-negative
MCF-7 cells [199]. Different concentrations of shikonin induce either apoptosis or necroptosis, and
necroptosis converts to apoptosis in the presence of Nec-1 in HL-60 and K562 cells
[200]. The growth inhibition and apoptosis induced by shikonin in some cancer cells may
be attributed to the inactivation of NF-κB activity or increasing Annexin V signal
and CD95 (Fas/APO) expression [201,202]. Shikonin also induces apoptosis via ROS production in osteosarcoma and Bcr/Abl-positive CML cells [203,204].

Emodin sensitizes chemotherapy associated with ROS production [221,222]. In combined use with cisplatin, emodin elevates ROS generation and enhances chemosensitivity
in DU-145 cells, accompanied by the down-regulation of MDR1 expression and suppression
of HIF-1α transactivation [223]. Emodin enhances the sensitivity of gallbladder cancer SGC996 cells to platinum drugs
via glutathione depletion and multidrug resistance-related protein 1 down-regulation [224]. The mechanisms of the synergistic effects of emodin with cisplatin or gencitabin
may be attributed to the emodin-induced down-regulation of ERCC1 and Rad51 expression,
respectively [225,226]. These results suggest that emodin may be used as an adjuvant to enhance the anti-cancer
effects of chemotherapeutic agents.

Ginsenoside Rg3

Extracted from Panax ginseng C.A. Mey. (Renshen) and Panax quinquefolius L., Araliaceae (Xiyangshen), ginsenoside Rg3 (Figure 1N) is a biologically active component with both in vitro and in vivo anti-cancer activities [227,228]. The anti-proliferative mechanism of ginsenoside Rg3 is associated with the inactivation of NF-κB [229,230], modulation of MAPKs [231] and the down-regulation of Wnt/β-catenin signaling [232]. Ginsenoside Rg3 affects the ephrin receptor pathway in HCT-116 human colorectal cancer cells [233]. The anti-proliferative mechanism of ginsenoside Rg3 is also associated with the molecules of mitotic inhibition, DNA replication, repair,
and growth factor signaling [234].

Ginsenoside Rg3 inhibits the proliferation of HUVEC and suppresses the capillary tube formation of
HUVEC on a matrigel at nanomole scales in the presence or absence of VEGF. Ginsenoside
Rg3 attenuates VEGF-induced chemo-invasion of HUVEC and ex vivo microvascular sprouting in rat aortic ring. bFGF-induced angiogenesis may be abolished
by ginsenoside Rg3 [227]. In lung metastasis models of ovarian cancer, ginsenoside Rg3 decreases the number of tumor colonies in the lung and vessels oriented toward the
tumor mass [235]. This effect may be partially due to the inhibition of angiogenesis and the decrease
in MMP9 expression [235].

Ginsenoside Rg3 increases the efficacy of cancer chemotherapy. Combined treatments with ginsenoside
Rg3 enhance the susceptibility of colon cancer cells to docetaxel, paclitaxel, cisplatin
and doxorubicin; the mechanism of such an enhancement is related to the inhibition
of the constitutively activated NF-κB [229]. A similar phenomenon has been observed in prostate cancer cells, in which the combination
of ginsenoside Rg3 and docetaxel more effectively induces apoptosis and G1 cell cycle arrest, accompanied
by the inhibition of NF-κB activity [230]. Low-dose administration of cyclophosphamide (CTX) with ginsenoside Rg3 increases the efficacy of targeting the tumor microvasculature and the two-drug combination
treatment results demonstrate the longest patient survival rates [236]. Ginsenoside Rg3 combined with gemcitabine not only enhances the efficacy of tumor growth suppression
and survival prolongation, but also decreases VEGF expression and microvessel density
in tumors [228].

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

WT, JJL, MQH, YBL, MWC, GSW, JG, ZFZ, ZTX, YYD and XPC wrote the manuscript (WT wrote
berberine; JJL wrote GA and ARTs; MQH wrote emodin and ginsenoside Rg3; YBL wrote
cucurmin; MWC wrote silibinin; GSW wrote shikonin; JG wrote wogonin; ZFZ wrote β-elemene;
ZTX wrote triptolide; YYD wrote UA; XPC wrote oridonin). JJG drew the chemical structures
in Figure 1. WT, JJL and XPC revised the manuscript. YTW designed and supervised this work. All
authors read and approved the final version of the manuscript.

Acknowledgements

The work was supported by the grant (029/2007/A2) from the Science and Technology
Development Fund of Macau Special Administrative Region, China and supported in part
by the National Natural Science of China (No. 81001450) awarded to JJL.

References

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